by Fred Espenak

Eye Safety During Solar Eclipses

The Sun can be viewed safely with the naked
eye only during the few brief seconds or minutes of a total
solar eclipse. Partial eclipses, annular eclipses, and the
partial phases of total eclipses are never safe to watch without
taking special precautions. Even when 99% of the Sun's surface
is obscured during the partial phases of a total eclipse, the
remaining photospheric crescent is intensely bright and cannot be
viewed safely without eye protection [Chou, 1981; Marsh, 1982].
Do not attempt to observe the partial or annular phases of any
eclipse with the naked eye. Failure to use appropriate filtration
may result in permanent eye damage or blindness!

Generally, the same equipment, techniques and precautions used
to observe the Sun outside of eclipse are required for annular
eclipses and the partial phases of total eclipses [Reynolds &
Sweetsir, 1995; Pasachoff & Covington, 1993; Pasachoff &
Menzel, 1992; Sherrod, 1981]. The safest and most inexpensive of
these methods is by projection, in which a pinhole or small opening
is used to cast the image of the Sun on a screen placed a
half-meter or more beyond the opening. Projected images of the Sun
may even be seen on the ground in the small openings created by
interlacing fingers, or in the dappled sunlight beneath a leafy
tree. Binoculars can also be used to project a magnified image of
the Sun on a white card, but you must avoid the temptation of using
these instruments for direct viewing.

The Sun can be viewed directly only when using filters
specifically designed for this purpose. Such filters usually have a
thin layer of aluminum, chromium or silver deposited on their
surfaces that attenuates ultraviolet, visible, and infrared energy.
One of the most widely available filters for safe solar viewing is
a number 14 welder's glass, available through welding supply
outlets. More recently, aluminized mylar has become a popular,
inexpensive alternative. Mylar can easily be cut with scissors and
adapted to any kind of box or viewing device. A number of sources
for solar filters are listed below. No filter is safe to use with
any optical device (i.e. - telescope, binoculars, etc.) unless it
has been specifically designed for that purpose. Experienced
amateur and professional astronomers may also use one or two layers
of completely exposed and fully developed black-and-white film,
provided the film contains a silver emulsion. Since all developed
color films lack silver, they are always unsafe for use in solar
viewing.

Unsafe filters include color film, some non-silver black
and white film, medical x-ray films with images on them, smoked
glass, photographic neutral density filters and polarizing filters.
Solar filters designed to thread into eyepieces which are often
sold with inexpensive telescopes are also dangerous. They should
not be used for viewing the Sun at any time since they often crack
from overheating. Do not experiment with other filters unless you
are certain that they are safe. Damage to the eyes comes
predominantly from invisible infrared wavelengths. The fact that
the Sun appears dark in a filter or that you feel no discomfort
does not guarantee that your eyes are safe. Avoid all unnecessary
risks. Your local planetarium or amateur astronomy club is a good
source for additional information.

In spite of these precautions, the total phase (and only the total phase) of an eclipse can
and should be viewed without filters. It is crucial that you know when to take off and put back on your glasses; see
Eye safety during a total solar eclipse